1. Field of the Invention
This invention relates to an optical device used as a super-wide image pickup lens that forms an image of a subject on a light receiving surface.
2. Description of the Related Art
A wide-angle lens that covers a wide field has been generally employed in on-board cameras and/or monitoring cameras. Accompanied with the advanced image processing technology, the super-wide lens capable of covering the wider field by correcting distortion of the image in real time has been increasingly demanded. Optical devices that satisfy requirements, such as compact size, high resolution, and low cost are very desirable, especially in view of the demands relating to the space for accommodating such devices and the required higher resolution of the picture element. Accordingly such devices also have application into other fields.
Specifically, the aforementioned optical system is expected to satisfy the following requirements:
Low cost (reduction in the number of lenses, use of the plastic lens, high moldability, easy assembly);
Brightness (small Fno);
Compact dimensions (overall length, lens diameter);
Wide field (wide field angle in excess of 180° depending on the application);
High resolution (Well corrected basic aberration);
Projection mode in need (isometric projection, three-dimensional projection)
Reduction in the number of lenses used for the optical device which satisfies the aforementioned requirements allows such optical device to be applied to various industrial fields.
FIG. 12 is a sectional view of the super-wide lens disclosed in Japanese Unexamined Patent Application Publication No. 4-68307. The super-wide lens including a lens system which covers a wider field angle and includes relatively a small number of lenses is formed of a front group and a rear group. As all the lenses of the lens system are spherical glass lenses, it is difficult to cover the field angle in excess of 90°, and accordingly, it is difficult to realize the projection with a small compression rate around the viewing surface, for example, isometric projection, three-dimensional projection and the like.
FIG. 13 is a sectional view of the super-wide optical system disclosed in Japanese Unexamined Patent Application Publication No. 2005-221920. The super-wide optical system is intended to solve the problem of the optical system as disclosed in Japanese Unexamined Patent Application Publication No. 4-68307 by employing the aspherical lens. It is structured such that the power of the front lens group is relatively stronger (magnification of the rear group is higher). Accordingly, the field angle equal to or smaller than 90° may be covered. If the field angle is intended to be further widened while using the plastic lens, it may increase the focal shift due to the temperature change as well as deteriorate the aberration balance between the front group and the rear group.
FIG. 14 is a sectional view of the super-wide lens as disclosed in Japanese Unexamined Patent Application Publication No. 2003-232998. The structure of the super-wide lens is based on that of Japanese Unexamined Patent Application Publication No. 4-68307 in which the aspherical lens is employed. Contrary to Japanese Unexamined Patent Application Publication No. 2005-221920, as the power of the front group of the super-wide lens is relatively lower, it is not suitable for the wider field angle. In the case where the plastic lens is employed, the focal shift due to the temperature change may be increased.